A wavelength division multiplexing (WDM) communication scheme has been under progress as an optical communication system for transmitting a plurality of optical signals. This system comprises erbium (Er) doped fiber amplifiers (EDFA) installed at predetermined locations along an optical path. Each EDFA is connected to a pumping laser module which has a semiconductor laser device as a pumping light source. Pumping laser light is directed from the laser module into the EDFA to optically amplify signal light transmitted from a signal light source, thereby transmitting the optically amplified signal light again to the downstream side.
In this event, a semiconductor laser device incorporated in the laser module is supplied with an injected current, the value of which is varied in response to fluctuations in the optical power of the signal light source to control the optical power of the pumping laser light.
Such a strategy is effective for a semiconductor laser device which has an emission wavelength in a 1480 nm wavelength range since a wide gain bandwidth can be provided in the EDFA. However, the strategy as mentioned above cannot be employed for a semiconductor laser device which has an emission wavelength in a 980 nm wavelength range since the gain bandwidth is narrow in the EDFA at this wavelength range. The present invention is directed to finding a way to overcome this limitation and to enable the use of 980 nm pumping light in EDFA applications.